As an exhaust gas purification system for purifying NOx in an exhaust gas from a diesel engine, an SCR system using an SCR device has been developed.
The SCR system supplies urea water to an exhaust gas upstream of the SCR device to generate ammonia using heat of the exhaust gas, and using the ammonia, reduces NOx on an SCR catalyst to purify NOx (see, for example, patent literature 1).
In the SCR system, injection of urea water is controlled in accordance with a NOx concentration in the exhaust gas. For this control, a NOx sensor is provided in an exhaust pipe.
If a detected value of this NOx sensor is not valid, the injection of urea water cannot be controlled appropriately, and thus, in the SCR system, a NOx sensor diagnosis device is provided for diagnosing validity of the NOx sensor.
In a prior-art NOx sensor diagnosis device, if a difference between a calculated value of the NOx concentration determined in accordance with an operation state of an engine and a detected value of the NOx concentration detected by the NOx sensor becomes larger than a predetermined threshold value, it is diagnosed that abnormality occurs in the NOx sensor (see, for example, patent literature 2).